The effect of structure and dynamics in model membrane systems on the excited-state and rotational reorientation behavior of merocyanine 540 (MC 540) will be studied by the method to time- resolved fluorescence depolarization. The long-term objective of the proposed research is to reveal the structure-reactivity relationships in the mechanism for the photochemotherapeutic activity of MC 540 in leukemia cells and viruses. Since cells are extremely complex systems, the first objective is to unravel these relationships in phospholipid vesicles, which are the simplest model membrane systems. The hypotheses which will be specifically tested in these studies are that (1) the fluid-state lipid bilayers preferentially bind MC 540; (2) the dye is more compact in fluid- state bilayers than in gel-state bilayers; (3) the photostability of the dye is greater in its elongated than compact form; and (4) the dye's ability to photoinduce damage in unsaturated liposomes is associated with the lower order in unsaturated liposomes and the dye's compact form. The excited-state decay law and the rotational anisotropy of MC 540 in micelles and in gel-state, fluid-state, mixed-phase, mixed headgroup, and mixed phospholipid-cholesterol vesicles will provide clues to these relationships, through information about dye-membrane interactions, the order of the local membrane environment, and the conformation of the dye molecule in that environment. The fluorescence decay and anisotropy in membranes will be analyzed in terms of multiexponential decay functions and a hindered-rotor model. Fluorescence measurements of MC 540 in solution will also be carried out to characterize the dye's excited-state and rotational reorientation properties in homogeneous isotropic media. To account for the solvent effects, a solvent-polarity dependent activation energy for the nonradiative rate constant and the Debye-Stokes-Einstein hydrodynamic equation for the rotational correlation time will be used.